1. Field of the Invention
The invention relates to a method of determining the rotational speed of a brushless dc-motor.
dc-motors are increasingly used in drive technology because they distinguish themselves by their high dynamic thrust and good controllability. Brushless DC-motors, i.e., electronically commutated DC-motors, moreover, offer the advantage that they operate almost without maintenance.
With brushless dC-motors, the position of the rotor with respect to the phase windings of the stator is determined by a rotor position generator transmitter. The phase windings of the stator are charged with current via switchable semiconductor valves of an intermediate circuit converter, in accordance with the position signals of the rotor position transmitter. The amount of the respective current which is supplied depends upon the desired torque of the motor. Therefore, the regulation of current generally underlies or forms a foundation for the rotational speed control. For the purpose of determining the rotational speed of the motor, a tachograph is additionally applied to the rotor, which constitutes an additional expenditure, just like the electronics required for transmitting the actual value of the rotational speed.
2. Description of the related Art including information disclosed under 37 CFR 1.97-1.99:
In German Published Non-Prosecuted Application (DE-OS) 38 20 903, a commutator-less DC-motor is described having a rotor bearing on its face an optically detectable signal trace for sensing the angular position of the rotor. In a three-phase motor, the signal trace is divided into three radially arranged individual traces which have three reflecting and three absorbing zones equally distributed over the circumference, the zones in the individual traces being offset 60.degree. electrically from one another. The traces are scanned by three radially arranged sensors, and the respective angular position of the rotor is determined for the purpose of commutation by the output signals of the sensors.
This published German application further proposes using the sensor signals for determining the rotational speed. In this regard, the six pulses generated by the rotor position transmitter per electric cycle, e.g., 18 pulses per armature revolution for a six-phase motor, are additionally used for determining the rotational speed. These 18 pulses per armature revolution, however, are insufficient for an exact control of the motor, especially at lower speed ranges. If a brushless DC-motor is used for the drive of printing machines, an exact determination of the rotational speed over the entire speed range must be assured, in order to attain optimal control performance. As mentioned hereinbefore, the mounting of an additional tachograph on the rotor is no alternative, because, in the manufacture of low-cost drives, every additional expenditure should be avoided.